Large sheets of materials having very flat surfaces, such as useful in making large flat panel displays or which can be diced to make other devices such as active electronic devices, photovoltaic devices, and biological arrays, are difficult to produce and handle. As an illustration, fusion draw processes, such as described in, for example, U.S. Pat. Nos. 3,338,696 and 3,682,609 issued to Dockerty, herein incorporated by reference, are known to deliver sheets of material having superior flatness and surface smoothness without the added expense of post-forming finishing operations such as lapping and polishing. However, it has been found that sheet motion in the forming zone can negatively impact the level of stress and stress variation within the sheet, possibly leading to distortion in the final product. Sheet motion in the forming zone may be attributed at least in part to sheet handling and separation processes at the bottom of fusion draw machine, or bottom of draw (BOD). For example, current mechanical processes for separating sheet at BOD involve sheet contact, sheet scoring, and sheet bending with hard or rigid devices, all of which contribute to sheet motion. The larger the sheet of material being formed, the more significant the effect of sheet motion may be on the stress level and variation within the sheet. The factors that contribute to distortion of sheets of material may take on different forms in other sheet forming and post-forming processes. To make large sheets with very flat surfaces, it is important to minimize these factors. Ideally, schemes to minimize these factors would take into account information about the shape profile of the sheet of material. It would be beneficial if such information can be readily obtained, either during sheet forming or post-forming processes.